Can Blender use the open-source Nouveau Nvidia driver for rendering?
CUDA only works with the Proprietary NVidia Driver. If you want to render using GPU you need to install it.

Nouveau doesn't support CUDA. Blender will run, and render, without CUDA, but it will not use the GPU.

Read: Can CUDA operate with open source Nvidia driver or only with Nvidia proprietary driver?

and the Nouveau article on wikipedia